I am trying to copy create (duplicate) the tab called 'predecessor' which is a one2many field . In the same, i want to map the subtask to the corresponding parent task

Code:

                pred_rec = self.env['project.task.predecessor'].search([('task_id', '=', task.id)])

                new_parent=self.env['project.task.predecessor']

        for predecessor_idrec in pred_rec:
                    for taskid in pred_rec:
                       if not taskid.parent_task_id:
                            new_parent=taskid   
                    for subtask in pred_rec:
                        if subtask.id!=new_parent.id:
                            subtask.write({'parent_task_id':new_parent.id})
                   predecessor_vals = {
                   'lag_type': predecessor_idrec.lag_type,
                    'parent_task_id': predecessor_idrec.parent_task_id,
                    'task_id': new_task.id,
                    'type': predecessor_idrec.type,
                     }
                   predecessor_rec = self.env['project.task.predecessor'].create(predecessor_vals)
  

parent_task_id  is the  name of the parent task field

I am getting the following error:

The operation cannot be completed, probably due to the following:
- deletion: you may be trying to delete a record while other records still reference it
- creation/update: a mandatory field is not correctly set

[object with reference: parent_task_id - parent.task.id]

This is because parent_task_id 's value is null

I am unable to solve the error